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LTC3251 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3251 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LTC3261 8 3261f APPLICATIONS INFORMATION Effective Open-Loop Output Resistance The effective open-loop output resistance (ROL) of a charge pump is a very important parameter which determines the strength of the charge pump. The value of this parameter depends on many factors such as the oscillator frequency (fOSC), value of the flying capacitor (CFLY), the nonoverlap time, the internal switch resistances (RS) and the ESR of the external capacitors. Typical ROL values as a function of temperature are shown in Figure 2 Figure 2. Typical ROL vs Temperature VRIPPLE(P-P) ≈ IOUT COUT • 1 fOSC –tON ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ where fOSC is the oscillator frequency tON is the on-time of the oscillator (1μs) typical and COUT is the value of the output capacitor. Just as the value of COUT controls the amount of output ripple, the value of CINcontrolstheamountofripplepresent at the input (VIN) pin. The amount of bypass capacitance required at the input depends on the source impedance driving VIN. For best results it is recommended that VIN be bypassed with at least 2μF of low ESR capacitance. A high ESR capacitor such as tantalum or aluminum will have higher input noise than a low ESR ceramic capacitor. Therefore, a ceramic capacitor is recommended as the main bypass capacitance with a tantalum or aluminum capacitor used in parallel if desired. Flying Capacitor Selection The flying capacitor controls the strength of the charge pump. A 1μF or greater ceramic capacitor is suggested for the flying capacitor for applications requiring the full rated output current of the charge pump. For very light load applications, the flying capacitor may be reduced to save space or cost. For example, a 0.2μF capacitor might be sufficient for load currents up to 20mA. A smaller flying capacitor leads to a larger effective open- loop resistance (ROL) and thus limits the maximum load current that can be delivered by the charge pump. Ceramic Capacitors Ceramic capacitors of different materials lose their capaci- tance with higher temperature and voltage at different rates. For example, a capacitor made of X5R or X7R material will retain most of its capacitance from –40°C to 85°C whereas a Z5UorY5Vstylecapacitorwillloseconsiderablecapacitance over that range. Z5U and Y5V capacitors may also have a poorvoltagecoefficientcausingthemtolose60%ormoreof theircapacitancewhentheratedvoltageisapplied.Therefore when comparing different capacitors, it is often more ap- propriate to compare the amount of achievable capacitance for a given case size rather than discussing the specified capacitance value. The capacitor manufacture’s data sheet Input/Output Capacitor Selection The style and value of capacitors used with the LTC3261 determine several important parameters such as regulator control loop stability, output ripple, charge pump strength and minimum turn-on time. To reduce noise and ripple, it is recommended that low ESR ceramic capacitors be used for the charge pump output. The charge pump output capacitor should retain at least 2μF of capacitance over operating temperature and bias voltage. Tantalum and aluminum capacitors can be used in parallel with a ceramic capacitor to increase the total capacitance but should not be used alone because of their high ESR. In constant frequency mode, the value of COUT directly controls the amount of output ripple for a given load current. Increasing the size of COUT will reduce the output ripple at the expense of higher minimum turn-on time. The peak-to-peak output ripple at the VOUT pin is approximately given by the expression: TEMPERATURE (°C) –50 20 60 50 55 40 45 35 30 25 50 25 0 100 75 –25 150 125 3261 F02 VIN = 12V VIN = 25V VIN = 32V fOSC = 500kHz |
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